The preparation of terephthalic acid by the air oxidation of liquid p-xylene in the presence of an acetic acid solution containing ions of bromine with cobalt and/or manganese ions was first disclosed in U.S. Pat. No. 2,833,816. But purification of terephthalic acid so produced by the catalytic hydrogenation of the oxidation effluent per se or diluted with acetic acid of from 0 up to 15 weight percent water so that all solids in the oxidation effluent dissolved at temperatures upward from 250.degree. C. was first disclosed in early 1967 in British Patent Specification No. 1,056,319. Said purification by hydrogenation used 3.5 to 14 kg/cm.sup.2 hydrogen partial pressure relied upon dissolved ions of cobalt and manganese to provide hydrogenation catalysis. The purified terephthalic acid, purified with respect to lowered 4-carboxybenzaldehyde (4-CBA) content, was recovered from the hydrogenated solution by recrystallization by cooling the solution to a temperature at least below the normal (760 mm Hg) boiling temperature of the solvent which was acetic acid containing from 3 up to 15 weight percent water. However, the tristimulus b-value color (a measure of yellowness indicated by a positive numerical value or of blueness indicated by a negative numerical value) of the purified terephthalic acid was not within the acceptable range of positive two to negative two, preferably positive one to negative one for fiber-grade quality terephthalic acid.
To be acceptable as fiber-grade terephthalic acid, that is a purified terephthalic acid acceptable for direct reaction with highly pure ethylene glycol in the manufacture of fibers from high molecular weight polyethylene terephthalate, the purified terephthalic acid had to have a purity of more than 99.9 weight percent and a 4-CBA content of 50 or less parts by weight per million (ppm) parts by weight of terephthalic acid in addition to the foregoing b-value color.
For the purified terephthalic acid to be used as an acceptable fiber-grade quality product from the standpoint of b-value color, total impurity and 4-CBA content, purification techniques other than those of the above British Patent Specification were needed. Such additional techniques included separating a partially purified terephthalic acid from the oxidation effluent by cooling it to a temperature of from 50.degree. to 100.degree. C. to maximize precipitation of terephthalic acid from solution, washing and drying the separated terephthalic acid. Such partially purified terephthalic acid was next dissolved in deionized water (U.S. Pat. No. 3,639,465) or in acetic acid containing from 0 up to 45 weight percent water (U.S. Pat. No. 3,546,285) at temperatures upward from 260.degree. C. to provide a solution containing 10 to 30 weight percent terephthalic acid and based thereon from 500 up to 6,000 ppm of 4-CBA. The solution and hydrogen gas are next contacted with a noble metal (Group VIII) as catalyst and, after separating the treated solution from the catalyst, highly pure terephthalic acid as fiber-grade product was recovered by recrystallization, washed with fresh solvent and the washed product dried. In commercial application the partially purified terephthalic acid was collected in one or more storage silos before being redissolved in water to prepare the solution for catalytic hydrogenation. Also the dried fiber-grade product was collected in one or more silos before use of shipment.
The terephthalic acid recovered from the oxidation effluent is a "partially purified" product because its separation from reaction effluent's mother liquor leaves behind 50 to 65 percent of the co- and by-product impurities and components of catalysis in the effluent's mother liquor, adhering mother liquor is washed away, and even wash liquor is removed by drying.
We have conducted successfully for more than 15 years on a commercial basis the foregoing combination of p-xylene oxidation, recovery and storage of partially purified terephthalic acid, redissolution of the partially purified product in deionized water and the subsequent steps of catalytic hydrogenation through washing and drying to arrive at the fiber-grade terephthalic acid product.
We also investigated redissolving in fresh acetic acid the partially purified terephthalic acid followed by the catalytic hydrogenation, separation from catalyst, recrystallization washing and drying to obtain fiber-grade terephthalic acid. In these investigations the catalyst was metallic palladium (0.5 and 1.0 weight percent) disposed on the surface of high surface area per unit weight activated carbon, the hydrogen partial pressure was 7 kg/cm.sup.2 and the hydrogenation temperature was 282.degree. C. and 310.degree. C. At the steady state conditions existing in a continuous flow process, using a 10 weight percent terephthalic acid solids content in predominantly acetic acid solvent, the 4-CBA concentration is about 150 ppm (dry basis) when using a 0.5 weight percent Pd catalyst at 282.degree. C. 4-CBA concentrations in the purification reactor effluent when using a 1 percent Pd catalyst are respectively, 190 ppm and 245 ppm at temperatures of 282.degree. C. and 310.degree. C. Increasing the concentration of terephthalic acid from 10 weight percent to 20 weight percent, using a temperature of 310.degree. C. and using a 1 percent weight Pd catalyst appears to reduce the 4-CBA concentration in the purification reactor effluent from 245 ppm to .about.210 ppm. Said weight percent of terephthalic acid were the solutions concentrations and each partially purified terephthalic acid contained 3,000 ppm 4-CBA. In contrast the same partially purified terephthalic acid but dissolved in water at 20 weight percent of the solution had at steady-state conditions 277.degree. C. and 7 kg/cm.sup.2 hydrogen partial pressure a 4-CBA concentration of 70 ppm and 76 ppm of 4-CBA at 304.degree. C. and 7 kg/cm.sup.2 hydrogen partial pressure. Those 4-CBA concentrations were for the total solids in the hydrogenated solution, hence the total 4-CBA.
Even more discouraging was the fact that acetic acid could be catalytically decomposed. Under the most severe conditions of 310.degree. C., when using a 20 weight percent terephthalic acid solution and catalyst of 1 weight percent palladium loading, total decomposition was determined to be 0.015 kg/kg terephthalic acid, and more terephthalic acid was converted to 4-CBA and p-toluic acid than when water was the reaction solvent. The acetic acid decomposition was mainly to carbon oxides, methane and ethane but analysis of the liquid phase showed acetaldehyde, acetone and methyl acetate as well as unknown products to be present.
Lastly, the publications of E. B. Maxted et al. concerning Catalytic Toxicity and Chemical Structure in the years 1937, 1938, and 1940 disclosed, based on the hydrogen gas reduction of N-crotonic acid at 25.degree. C. in alcohol or acetic acid solution (10 ml.) and platinum (0.05-0.1 g) as catalyst, that many dissolved metals (in amounts between about 0.8 and about 1.6.times.10.sup.-6 gram atoms) were tested for their toxic effect caused by being absorbed as layer or layers on the catalyst. The authors reported copper, silver and tin to have the same lowest deactivation of catalytic activity which for comparative purposes was assigned the relative effective toxicity of 1.0. With respect to the other metals tested their relative toxicities were compared to that of copper. Such toxicities were reported shown in Table I to follow which is from Table VI of the authors' 1940 publication.
TABLE I ______________________________________ RELATIVE TOXICITIES OF METALS AGAINST PLATINUM AT 25.degree. C. Metal Relative Toxicity ______________________________________ Copper 1.0 Mercury 1.7 Thallium 2.8 Lead 3.7 Zinc 4.0 Cadmium 4.0 Nickel 3.7 Manganese 4.0 Iron 4.1 Cobalt 5.1 ______________________________________
Such 1940 reported test results confirmed earlier work of Paal et al. (Ber. 44, 46 and 51 in, respectively 1911, 1913, and 1918) that the metals copper, mercury, lead, zinc, cadmium deactivated platinum, and palladium catalysts when such metals were the support for said catalysts.
The foregoing toxicity facts concerning the relative toxicity of cobalt and manganese being, respectively, 3 and 2.4 times more toxic than mercury and even more toxic (cobalt being 1.38 more toxic) than lead and nickel appeared to indicate failure for our concept of using hydrogenation in the presence of palladium or platinum as catalyst of the p-xylene oxidation effluent as a step in the production of fiber-grade terephthalic acid.
However, in spite of the somewhat negative results obtained by the catalytic hydrogenation of fresh acetic acid solution of partially purified terephthalic acid and the apparent forecast of failure of our purification concept, we were indeed surprised with the success of an actual application of our inventive concept. We were able to decrease the 4-CBA content down to less than 50 ppm from 3,000 ppm, without excessive p-toluic acid appearing in the product and provide a product having a low b-color value below positive 1.0. Such results would not have been expected from a process whose hydrogenation step had present for each 0.1 gram of hydrogenation catalyst metal more than 10 times the amount of cobalt and more than 20 times the amount of manganese tested by Maxted et al. and found rather deactivating. Our amounts of cobalt and manganese and the amounts of the same metals tested by Maxted et al. per 0.1 gram hydrogenation catalyst metal are shown in Table II which follows.
TABLE II ______________________________________ Gram-Atom X10.sup.-6 of Cobalt and Manganese Per 0.1 Gram of Catalyst Metal Inventive Metal Maxted et al. Concept Times Greater ______________________________________ Cobalt 0.8 to 1.6 160 200 to 100 Manganese 0.8 to 1.6 320 400 to 200 ______________________________________
The results of the example of the novel hydrogenation step of the present inventive concept will demonstrate, when measured against the results of the comparative example, that the presence of such gross amounts of cobalt and manganese during the platinum family metal catalyzed hydrogenation had little catalytic deactivating effect. Such amounts of cobalt and manganese may have offset the observed high p-toluic acid formation by over-hydrogenation of terephthalic acid as well as the high 4-CBA reduction floor (140 to 245 ppm total 4-CBA) when partially purified terephthalic acid was dissolved in fresh acetic acid and the solution hydrogenated in the presence of the platinum family metal catalyst.